tests: add test/performance comparison for mixing special-case code

tests: add test/performance comparison for mixing special-case code

x86-64, i7-870, 3 GHz gcc -O0, 100 TIMES

mix s16 generic 1 channel: 286503 usec (avg: 2865.03, min = 2673, max = 6892, stddev = 425.996).
mix s16 2 streams 1 channel: 136490 usec (avg: 1364.9, min = 1304, max = 1686, stddev = 73.4594).

mix s16 generic 2 channels: 590472 usec (avg: 5904.72, min = 5342, max = 16027, stddev = 1079.34).
mix s16 2 channels: 380369 usec (avg: 3803.69, min = 3570, max = 5041, stddev = 234.831).
mix s16 2 streams: 316254 usec (avg: 3162.54, min = 3034, max = 3536, stddev = 107.716).
mix s16 2 streams 2 channels: 269645 usec (avg: 2696.45, min = 2585, max = 3027, stddev = 87.9661).

x86-64, i7-870, 3 GHz gcc -O2, 1000 TIMES

mix s16 generic 1 channel: 371550 usec (avg: 3715.5, min = 3515, max = 10534, stddev = 760.071).
mix s16 2 streams 1 channel: 207750 usec (avg: 2077.5, min = 2009, max = 2271, stddev = 58.6076).

mix s16 generic 2 channels: 724294 usec (avg: 7242.94, min = 6937, max = 10350, stddev = 363.451).
mix s16 2 channels: 345661 usec (avg: 3456.61, min = 3291, max = 5586, stddev = 256.309).
mix s16 2 streams: 559243 usec (avg: 5592.43, min = 5349, max = 6705, stddev = 223.271).
mix s16 2 streams 2 channels: 390101 usec (avg: 3901.01, min = 3696, max = 5345, stddev = 213.482).

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>